Recently, owing to the demand for data communication and an increase of various communication services in a wireless communication system, various methods for increasing transmission capacity are being proposed. In one method, a technology for heterogeneous network is being researched. The heterogeneous network represents a communication system in which Base Stations (BSs) having various prestige, cell coverage, and characteristics are managed in combination.
FIG. 1 illustrates an example of a construction of a heterogeneous network system. Referring to FIG. 1, a macro cell 100 and one or more pico cells 102, 104, and 106 having cell coverage of different sizes are overlaid and managed. Also, although not illustrated in FIG. 1, at least one femto cell can be further included within the macro cell 100 of the heterogeneous network. For description convenience below, a pico cell and a femto cell are commonly called a ‘small cell’ in the present disclosure.
The heterogeneous network can reuse a transmission resource in a small cell existing within a macro cell and increase the whole transmission capacity of the system. For example, in a situation where one small cell having a transmission band of 10 Mega Hertz (MHz) is installed in the macro cell in an environment using a 10 MHz transmission band, the total transmission band of 20 MHz can be secured ideally. That is, in a situation where ‘N’ small cells are installed in one macro cell, a transmission band corresponding to ‘N’ times of an existing transmission band can be secured ideally. In other words, the effect of increasing a transmission rate as much as ‘N’ times can be expected.
However, in the real environment, although ‘N’ small cells can be installed within one macro cell, the cells may fail to get a transmission rate of ‘N’ times because of interference between heterogeneous cells or non-uniform distribution of terminals. That is, in a real heterogeneous network system, interference between the macro cell and the small cell and interference between the small cells can lead to the occurrence of performance degradation. Also, the non-uniform distribution of terminals in the small cells can lead to generation of unavailable resources, causing performance degradation.
To minimize the performance degradation resulting from inter-cell interference and loading unbalance in the wireless communication system including the heterogeneous network, a range expansion technique of effecting an increase of the coverage of a small cell can be used in cell selection. The range expansion technique is a technology for adding a constant biased value to a channel quality of a corresponding cell at channel quality comparison for cell selection, thereby increasing a possibility that the cell will be selected. However, in reality, a terminal may select a cell of a poor channel quality as a serving cell because the biased value is added to the channel quality at cell selection. In a situation where the terminal selects the cell as the serving cell despite the poor channel quality, a situation can occur in which the terminal cannot receive even a control signal, a broadcast channel and the like.
Accordingly, an alternative for ensuring a minimum communication quality when a terminal located in an area applying the range expansion technique attempts initial access should be presented.